An integrated circuit may contain a high voltage n-channel metal oxide semiconductor (MOS) power transistor which is configured to operate at a drain voltage which is significantly higher than an operating voltage for other transistors and circuits in the integrated circuit. For example, an integrated circuit which contains transistors and circuits which operate at 10 volts or less may also include a high voltage n-channel MOS transistor which operates at a drain voltage of over 300 volts and switches several amps. The body of the high voltage transistor may be directly connected to the substrate of the integrated circuit, for example to provide a simpler fabrication process for the integrated circuit, compared to an integrated circuit with an isolated high voltage transistor. It may be desirable to determine if current through the high voltage transistor is above a certain value when the high voltage transistor is in the on state, without increasing the fabrication complexity of the integrated circuit or unduly increasing the size of the integrated circuit.